Method and system for determining static and/or dynamic, loads using inverse dynamic calibration

Abstract

Methods and systems are described for the identification and determination of loads by inverse analysis. A new combination of instrumentation and measurements is provided which allows accurate identification/measurement of static and/or dynamic forces, or loads, working on arbitrary test-objects or systems (i.e. test objects), through measurements of the results of the loads on the test object, signals like pressures, electrical potential, magnet flux displacement, strain, etc. This procedure allows the measurement/identification of arbitrary single or multiple loads, in arbitrary combinations, and multiple directions. The procedure also allows the measurement/identification of loads with arbitrary evolution in time, including static and dynamic loads.

The invention claimed is: 
     
       1. A method of determining low frequency or static loads on a system, the system comprising a test-object or structure operating in a first mode of operation, the method comprising:
 measuring indicators representing results of first loads exerted on at least one element of the test-object or structure during a first time interval when the system or structure is operating in the first mode of operation, and 
 deriving by measurement a relation between the first loads and the indicators valid for the first mode of operation in a controlled way during a second time interval, 
 wherein during the second time interval the test-object or structure is operated in a second mode of operation in which the test object or structure is conditioned to allow measurement of responses to applied known, or measureable, second loads, the environment of the test-object or structure and/or the way the test-object or structure is supported, is/are changed compared to the first mode of operation, the relation being determined by applying second loads as dynamic excitations and measuring responses thereto at frequencies which are higher then the frequencies of the first loads applied in the first mode of operation, 
 extrapolating, or completing down in frequency the relation to cover the frequency range relevant for the first mode of operation, the extrapolation being made either by assuming a frequency independent relation below the measured frequency range or based on a modal model of the test-object or structure, and being applied in an inverse analysis in order to calculate the first loads at other frequencies in the time domain or in the frequency domain. 
 
     
     
       2. The method of  claim 1 , wherein part of the frequency range is overlapping in frequency of the first and the second time intervals. 
     
     
       3. The method of  claim 1 , wherein during, the second time interval, the measurement is done on a partially disassembled test-object or structure, when loads at certain interfaces within the test-object or structure are to be identified, and/or during, the second time interval, the measurement is done on the test-object or structure, or remaining part of, in boundary conditions which provide negligible constraining dynamic forces from the environment, or supports, in the frequency range used in the second time interval, this boundary condition limiting a range of static displacement. 
     
     
       4. The method of  claim 1 , wherein the dynamic constraining forces from the environment are not negligible, causing additional suspension modes in the frequency range of the second time interval, and wherein a modal model is used to extract the unconstrained behavior from the measured relations between the loads and the indicator signals. 
     
     
       5. The method of  claim 1  wherein the extrapolation includes a determination from measurements of a relation matrix between first loads and indicators for the inverse analysis. 
     
     
       6. The method of  claim 1 , wherein the indicators being results of the loads are strains. 
     
     
       7. The method of  claim 1 , wherein the test-object or structure to be tested is a vehicle. 
     
     
       8. The method of  claim 1  wherein the second loads, as used during the second time interval, are not in the same direction, or same location, as the loads in the first time interval, and geometrical transformation is used to derive the required relations for the identification of the loads in the first time interval. 
     
     
       9. The method of  claim 1  wherein the measured relation between the loads and indicators thereto carried out during the second time interval, is synthesized using identified modes and upper and lower residual terms, optionally in combination with integration or differentiation in the frequency domain. 
     
     
       10. A non-transitory machine readable storage medium for storing a computer program product, which when executed by a processor, performs the method of determining low frequency or static loads on a system, the system comprising a test-object or structure operating in a first mode of operation, the method comprising:
 measuring indicators representing results of first loads exerted on at least one element of the test-object or structure during a first time interval when the system or structure is operating in the first mode of operation, and deriving by measurement a relation between the first loads and the indicators valid for the first mode of operation in a controlled way during a second time interval, wherein during the second time interval the test-object or structure is operated in a second mode of operation in which the test object or structure is conditioned to allow measurement of responses to applied known, or measureable, second loads, the environment of the test-object or structure and/or the way the test-object or structure is supported, is/are changed compared to the first mode of operation, the relation being determined by applying second loads as dynamic excitations and measuring responses thereto at frequencies which are higher then the frequencies of the first loads applied in the first mode of operation, extrapolating, or completing down in frequency the relation to cover the frequency range relevant for the first mode of operation, the extrapolation being made either by assuming a frequency independent relation below the measured frequency range or based on a modal model of the test-object or structure, and being applied in an inverse analysis in order to calculate the first loads at other frequencies in the time domain or in the frequency domain. 
 
     
     
       11. A system for the determination of low frequency or static loads on system, the system including at least a test-object or structure, first loads being exerted on at least one element of the test-object or structure during a first time interval,
 the system comprising: 
 first means for measuring indicators being the results of application of the first loads exerted on at least one element of the test-object or structure during the first time interval of operating in a first mode of operation, 
 means for exerting second loads in a controlled way on at least one element of the test-object or structure during a second time interval of operating in a second mode of operation, 
 second means for measuring responses to the application of the second loads, 
 means for deriving the relation between the first loads and the indicators, the means for deriving using outputs of the second means for measuring, 
 wherein during the second time interval the test-object or structure is conditioned to allow measurement of the responses, the environment of the test-object and/or the way to support the test-object being changed compared to the first mode of operation to allow measurement of responses to applied known, or measureable, second loads, the second means for measuring being arranged to measure the responses at frequencies which are higher then the frequencies of the loads in the first mode of operation, 
 the means for deriving being arranged to extrapolate the responses, or complete down in frequency, to thereby cover the frequency range of the first mode of operation and to extrapolate either by assuming a frequency independent relation below the measured frequency range or based on a modal model of the test-object or structure, and being arranged to apply an inverse analysis to calculate the first loads in the time domain or in the frequency domain. 
 
     
     
       12. The system of  claim 11 , wherein a part of the frequency range is overlapping in frequency of the first and the second time intervals. 
     
     
       13. The system according to  claim 11  being a computer based system. 
     
     
       14. The system according to  claim 11  wherein the first or second means for measuring comprise strain gauges. 
     
     
       15. The system according to  claim 11 , wherein the means for deriving is arranged to determine from measurements a relation matrix for the inverse analysis. 
     
     
       16. The system according to  claim 11  wherein during the second time interval the test-object or structure is a partially disassembled test-object or structure, when loads at certain interfaces within the test-object or structure are to be identified, and/or during, the second time interval, the measurement is done on the test-object or structure, or remaining part of, in boundary conditions which provide negligible constraining dynamic forces in the frequency range used in the second time interval, but wherein the boundary condition limits the range of static displacement. 
     
     
       17. The system of  claim 11 , wherein the dynamic constraining forces from the environment are non-negligible, causing additional suspension modes in the frequency range of the second time interval, and wherein the system is arranged to utilize a modal model to extract the unconstrained behavior from the measured relations between the loads and the indicator signals. 
     
     
       18. The system according to  claim 11 , wherein the test-object to be tested is a vehicle. 
     
     
       19. The system according to  claim 11 , wherein the excitation forces during the second time interval are not in the same direction, or same location, of the loads in the first time interval, and geometrical transformation is used to derive the required relations for the identification of the loads in the first time interval. 
     
     
       20. The system according to  claim 11 , wherein the system is arranged so that a measured relation between the loads and indicators thereto carried out during the second time interval, is synthesized using identified modes and upper and lower residual terms.